Glass mix sharpens holograms

By Ted Smalley Bowen, Technology Research News

The concept of storing information as holograms has been around for decades, but the lack of a suitably thick and stable material for storing holograms has hindered its development.

Given the right material, however, holograms could store more than 20 times as much information per square inch as standard magnetic or optical media. The data could be retrieved faster, as well.

A pair of researchers from Canada and Spain have made a glass composite that is a good candidate for the job. It can be made at least a millimeter thick, resists distortion, and has the optical properties needed to store holograms, according to the researchers.

The modified silica "makes holographic storage a more viable alternative to today's storage media including CD-ROMs and DVDs," said Pavel Cheben, a senior member of the research staff at Optenia, Inc.

A holographic disc of the material the size of a standard CD, for instance, can hold nearly a terabit of data, or about 20 times as much per square inch as a single-layer DVD, he said. This translates to about one billion pages of double spaced text or one million high-resolution photographs.

Such high data densities are possible with holographic storage because individual pages of data can literally be stored in the same space.

To store data holographically, a spatial light modulator first converts the ones and zeros of digital data into a string of light and dark areas and displays them on its screen. A laser beam is then split into an object beam and a reference beam. The object beam is shined through the screen, picking up the light and dark areas of the binary image as part of its wave pattern.

When the reference beam converges with the object beam on a light-sensitive storage material, the two streams of light interfere with each other, creating a pattern of light and dark areas that is captured in the material.

Light interference occurs when the peaks and troughs of two lightwaves meet. Where a pair of peaks or troughs line up they reinforce each other and and the light becomes brighter. Where a peak and a trough line up, they cancel each other out and the light becomes dimmer.

Subsequent pages of data can be recorded as holograms in the same space by slightly varying the angle of the reference beam, essentially creating a three-dimensional stack of two-dimensional page images. The thicker the storage medium, the more pages of data it can contain.

Because a whole page of data is stored in each image, the whole page can be retrieved at once. This makes data retrieval times much faster than those possible with current storage media, which retrieve information serially, one bit at a time.

Demonstrated data retrieval times from holographic images are as fast as one gigabyte per second, which is more than 400 times faster than today's 2.4-megabyte-per-second, 16-times CD-ROMs, according to Glenn Sincerbox, a professor of optical sciences at the University of Arizona.

To produce the material, the researchers used a sturdier, inorganic glass base in place of the organic polymers used in previous experiments. They made the photosensitive glass composite by adding a photoinitiator and an acrylic to a porous silica.

The researchers mixed the components, then cast them in Teflon vials. The results were one-millimeter-thick slabs of optical-grade glass composite.

The photoinitiator triggers polymerization, or hardening, of the acrylic in the areas of the composite hit by light, in particular the light parts of the interference pattern from the laser beams.

During polymerization, individual molecules link up with each other. Because the silica is porous, molecules of the liquid acrylic are drawn from the dark areas to the light areas by the polymerization process. The result is a much higher concentration of the acrylic in the areas exposed to light.

The acrylic has a higher refractive index than the surrounding glass, meaning it deflects light at a sharper angle. Data is retrieved from a holographic storage medium by shining a laser beam through it at the angle of the reference beam when that particular page of data was stored. The positioning of the acrylic makes the light refract into the original pattern.

The large difference between the refractive index of the glass base and that of the acrylic gives the material a strong refractive index modulation, which allows it to store data pages at resolutions as high as 1,024-by-1,024-pixels, said Cheben. It also means an area of the material a little over a millimeter square could potentially hold as many as 1,000 pages, he said.

"The stronger the refractive index modulation is, the more efficient holographic imprinting is... more holograms can be recorded and retrieved with higher signal strength, which ultimately results in a higher storage density and less noisy readout," he said.

The photoinitiator the researchers used made the medium more sensitive to blue and green light, which is more efficient than colors at the other end of the spectrum. "High photosensitivity means that less light power is needed to record a hologram, or, using the same light power... a shorter time. Sensitivity in blue and green is preferred because [they have] shorter wavelengths than red light, and data storage density increases with decreasing wavelength," Cheben said.

The researchers have stored plane wave grating images in the material for six months, according to Cheben. Plane wave gratings are prism-like devices used to separate individual colors from light, a process that produces simple patterns on surfaces. The researchers' next step is testing the material with actual data, he said.

The initial tests showed "excellent holographic properties of the glass including dynamic range, light sensitivity, shrinkage, and scattering," Cheben said. The tests with actual data should show the same, he said.

The medium's optical quality and stability so far are promising, said Sincerbox.

One trade-off, however, results from the researchers' use of a relatively low concentration of the photoinitiator to ensure uniform light absorption throughout the glass. The low concentration means a correspondingly low number of holograms can be superimposed, said Sincerbox. Holographic storage can theoretically store 2,000 to 5,000 pages in one area, Sincerbox said.

Commercial implementations of memory devices made from the composite could be feasible in two or three years, according to Cheben.

Cheben worked on the holographic storage project while with the Canadian National Research Council. His research colleague was Maria Calvo of Complutense University in Madrid.

The researchers described the work in the March 12, 2001 edition of the journal Applied Physics Letters. The research was funded by Complutense University.

Timeline:  2-3 years
Funding:  University
TRN Categories:  Data Storage Technology
Story Type:  News
Related Elements:   Technical paper: "A photopolymerizable glass with diffraction efficiency near 100% for holographic storage," Applied Physics Letters, March 12, 2001.


April 11, 2001

Page One

Glass mix sharpens holograms

Material bends microwaves backwards

Shaky chip makes for bug-eyed bots

Cold plastic gives electrons free ride

Holographic technique stresses interference


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